A variety of metering devices are used to measure the flow of streams of fluid, liquid and gas. Any such metering device necessarily impedes the flow to some extent. Such interference, at least in forced flow through pipes, is invariably undesirable. Since the transmission of fluid through pipes requires energy input by pumps, compressors or the like, the loss of pressure resulting from the flow of a stream of fluid through a metering device is costly and therefore desirably minimized.
The invention herein described relates to the type of flow meter which employs a primary flow element that creates a fluid pressure differential that is proportional to the rate of flow of the fluid so that measurement of the pressure differential indicates the rate of flow. Since the invention is primarily concerned with the pressure loss problem, it contemplates primary flow elements of the flow tube type rather than orifice plates and nozzles which are characterized by relatively great pressure losses.
A typical flow or Venturi tube comprises an inlet section which reduces the cross-sectional area of the flowing stream from that of the upstream pipe to that of the throat section, at which the flow area is the smallest, and a generally conical pressure recovery or diffuser section which leads back toward or to the full area of the downstream pipe.
A considerable amount of work has been done in efforts to reduce pressure losses which are concomitant with the flow of fluid streams through metering flow tubes. Almost invariably the approach to this problem has been directed to the pressure recovery section of the device downstream from the throat. The concensus of scientific thinking appears to have been that energy losses were related to stream/wall interface phenomena as the cross-sectional area of the flowing stream was returned to that of the transmission pipe. A typical statement is that of Prof. Leslie J. Hooper in his excellent discussion entitled, Design and Calibration of the Low-Loss Tube, ASME Paper No. 61-WA-80, presented Nov. 26, 1961: "It is well known that the losses due to contraction of sections are almost negligible while the losses in expanding sections are far greater. For this reason the analysis of the diffuser was undertaken in order to secure a low loss in this tube design."
Functional design features of the flow tubes herein described conform, generally, to the guidelines of Prof. Hooper's paper. However, major improvement with respect to pressure loss characterizes the tubes of the instant invention by reason of a novel approach to the problem involving the design of the throat inlet section. The contour of this section in the improved flow tube is such that the acceleration of flow of the fluid throughout the length of the inlet section is constant. It has been found that the transition in flow velocity from that in the pipe to the maximum at the throat of the tube, effected at a constant rate of acceleration, is of such a fully controlled nature that losses are of a very much lower order of magnitude than are the losses experienced with flow tubes having inlet section contours generally in use; e.g., those which are circular in profile.
It is, accordingly, an object of this invention to provide fluid flow metering tubes which impose only very low pressure losses on flow through the meter; more specifically, the invention is directed to the provision of a novel throat inlet section for use in primary flow tube elements to minimize pressure loss, the inlet section having a flow controlling contour which results in acceleration of flow at a constant rate.
A further object is to provide a flow tube throat inlet section structure which can be fabricated with faithful adherence to contour design specification and which, also, is not subjected to mechanical stress in the installed assembly so that calibration is maintained through a long period of use.
More specifically, the throat inlet section may be formed of thin metal which is attached to and supported by only the tube throat section structure without engagement with the stress bearing structure of the meter.